Pulmonary arterial hypertension (PAH), a progressive fatal disease, manifests by vascular remodeling of pulmo- nary arteries (PA), elevated right ventricular afterload, right heart failure and death. A key component of PA remodeling is the progressive vessel wall thickening due to increased proliferation and impaired apoptosis of pulmonary arterial vascular smooth muscle cells (PAVSMC). We previously identified mTOR signaling as a key positive regulator of proliferative/apoptosis-resistant PAH PAVSMC and provided pre-clinical data for a clinical trial to test mTOR inhibitor ABI-009 for patients with PAH. Recently, bi-directional cross-talk between PAVSMC and extracellular matrix (ECM) composition and stiffness was identified as a key regulator of pulmonary vascular remodeling and PAH. The mechanism(s) linking ECM, mTOR-supported PAVSMC remodeling, and proliferation of other resident PA cells are not known, and safe and easily translatable therapeutic options remain to be determined. We have found that growth suppressor TSC2, a key inhibitor of mTOR complex 1 (mTORC1), also acts as a mechanosensor and forms negative bi-directional cross-talk with mechanotransducers Yap/Taz, hyper- activation of which in PAH promotes ECM stiffening and pulmonary vascular remodeling. We report that TSC2 is deficient in PAVSMC from subjects with PAH, resulting in activation of mTORC1, Yap/Taz, consequent up- regulation of pro-survival mTORC2-Akt, increased proliferation, survival, and over-production of ECM proteins. ECM, produced by human PAH PAVSMC, activates mTORC1, and up-regulates Yap/Taz, mTORC2-Akt and PAVSMC proliferation, as well as growth of PAEC and PAAF. Last, the restoration of TSC2 by Sirt1 activator SRT2104 down-regulates both Yap/Taz and mTORC1/2, reduces ECM production, inhibits proliferation and in- duces apoptosis in human PAH PAVSMC, and reduces SU5416/hypoxia PH in mice. During the next funding period, we propose to elucidate the role of TSC2 as a critical coordinator of ECM-pulmonary vascular cell inter- actions and explore the benefits of TSC2 restoration by a novel well-tolerated drug with high translational poten- tial to correct deregulation of both mTOR and Yap/Taz networks, and reverse PA remodeling and PH. Specifi- cally, we will (1) critically test the status of TSC2, its relationship with Yap/Taz, mTORC1 and 2, and its role in PAVSMC proliferation, survival, pulmonary vascular remodeling and PH using de-identified lung tissues and cells from PAH and healthy subjects and mice with VSM-specific Tsc2 knockout; (2) determine whether TSC2 deficiency in PAVSMC is induced by increased matrix stiffness and self-supported via a Yap/Taz-dependent ECM loop, and evaluate the role of TSC2 in ECM remodeling and ECM-dependent proliferation of PAEC and PAAF; and (3) test whether restoration of TSC2 by SRT2104 suppresses PAH PAVSMC proliferation, induces apoptosis, reduces pathological ECM production and consequent hyper-proliferation of PAEC and PAAF, and reverses pulmonary vascular remodeling and PH. The proposed study will identify new critical mechanisms of pulmonary vascular remodeling and dissect new important molecular target for therapeutic intervention.